Improving the efficiency or the specific power of a gas turbine or a combustion engine requires, among others, an increase in combustion gas temperatures. For example, for gas turbines, the efficiency can be improved by increasing the turbine inlet temperature. However, the maximum exhaust gas temperature is normally limited by the materials used to fabricate the vanes and blades in the high pressure turbine part of the engine. Today, nickel-base superalloys are used in high pressure turbines which are capable of service temperatures up to about 1150° C. for long-term applications (several thousand hours). Although these temperatures more or less mark the upper temperature limit for the class of superalloys with melting ranges of from about 1230 to 1400° C., the gas temperatures in current engines can be raised by a variety of techniques, such as improved cooling technologies and thermal barrier coatings.
As a consequence of higher exhaust gas temperatures in high-pressure turbines, the other components of the gas turbine engine must operate at higher temperatures as well. These components include rotating and static parts, such as high pressure compressor airfoils, low pressure turbine airfoils and combustors.
The materials used for these components must usually meet low weight and increased operating temperature requirements. Therefore, in particular for aircraft engine and automotive applications, titanium alloys and titanium aluminides have been developed and introduced into such components. For stationary gas turbines, weight reduction is less a driver for materials development and/or replacement, however, the benefit obtained from light weight materials is obviously based on reduced forces placed on the shafts by using light weight rotating components (e.g. blades).
At moderately elevated temperatures of from about 500 to 900° C., the alloys used are exposed to severe environmental attack during service. Depending on the temperature and the operating environment (air, combustion atmosphere, particle-loaded gas, solid/solid friction), the modes of attack include hot corrosion, oxidation, erosion, and wear. Most alloys selected for good mechanical properties and low weight are susceptible to these types of attack.
It is therefore necessary to protect the components from environmental attack. One common approach is to use protective coatings to resist the modes of damage mentioned above while using the mechanical properties of the structural material forming the component.
Although numerous coating systems designed for protection of high-temperature alloys could be used for e.g. titanium alloys, these coatings do not meet the requirements for a variety of reasons. E.g. coatings used for protection of nickel-base superalloys, such as MCrAly-type overlay coatings, form brittle phases and degrade the mechanical properties when applied to titanium alloys and titanium aluminides. Furthermore, interdiffusion between the MCrAlY-type overlay coating and the titanium alloy or titanium aluminides substrate alloy can lead to several other modes of degradation such as pore formation at the coating-substrate interface, loss of protectiveness of the coating, reduced lifetime of the coating and the entire system, etc. Other coating systems that have been investigated in the past, such as oxide ceramic coatings, are inherently brittle and thus degrade the mechanical properties of the titanium alloy or the titanium aluminide substrate material, particularly fatigue resistance. Furthermore, with increasing gas temperatures in gas turbines of combustion engines there might be the need for thermal insulation of components such that these can be used at temperatures exceeding the limit given by their mechanical properties.
Thus, there is a need for an environmental and thermal protection system, in particular for titanium alloys and titanium aluminides to be used at moderately elevated temperatures, particularly in hostile environments.